Railroads carry large amounts of aggregate lading such as coal, stone, ores and the like in open top hopper cars having gravity discharge openings in the car body bottom which are closed by openable doors or gates. Such doors were originally opened manually but, to reduce costs and speed lading discharge, various systems were developed to open the doors by mechanical means of different types. These discharge systems usually required that the car be stationary before the doors were opened. While such systems are highly useful for many shippers, they are still undesirably slow for unloading unit trains, such as trains of one hundred coal-loaded hopper cars. The unloading of trains of that magnitude is best effected by discharging the lading by opening the doors of a car in car-by-car sequence as the train continuously moves past the unloading site. The unloading of hopper cars in this manner requires that the door operating mechanism be safe and reliable and respond quickly to suitable signals to which the door operating mechanism responds. To prevent the door operating mechanism from opening accidentally a secondary lock is generally included. Such secondary locks are disclosed in U.S. Pat. Nos. 3,710,729 and 4,132,177. Also, U.S. Pat. Nos. 3,596,608 and 3,596,609 disclose door-actuating mechanisms.
Although the previously available apparatus for operating the hopper doors is useful for the intended purpose, alternative apparatus which operates the doors quickly and reliably and provides easy adjustment so that all doors can be tightly closed is desirable, as well as an improved secondary lock mechanism.